A recently developed vacuum tube for handling r.f. signals includes a cathode for emitting a linear electron beam, a grid positioned at right angles to the direction of flow of the beam in close proximity to the cathode (no farther than the distance an emitted electron can travel in a quarter of an r.f. cycle at the highest frequency being handled by the tube) for current modulating the beam, and a cavity resonant to the frequency of the signal positioned between the grid and a collector electrode for the beam. The grid is coupled by a structure resonant to the frequency being handled by the tube to an r.f. input signal to be amplified by the tube. To prevent electron emission from the grid, it is formed of a non-emissive material, such as pyrolytic graphite or molybdenum coated with zirconium.
As applied to the electron beam flowing beyond the grid, the terms "current-modulated," "space-charge-modulated," "density-modulated" and "intensity-modulated" are synonymous, and refer to concentrations (or "bunches") alternating with depletions of particle density (or space-charge density) along the beam. Speeding and slowing of particle velocity is indicated by the term "velocity modulation."
Very high efficiency is achieved with such a tube by biasing the grid so that current flowing from the cathode toward the grid occurs for no more than one half cycle of the r.f. signal handled by the tube. Typically, the bias voltage between the grid and cathode is very small or zero.
In one prior art configuration, the resonant input circuit supplies electric fields having opposing phases between the cathode and grid and between the grid and an accelerating anode positioned between the grid and the output cavity. In another prior art modification, a second resonant cavity positioned between the output cavity and the accelerating anode is adjusted so the resonance frequency thereof is above the frequency being handled by the tube, to increase the average efficiency of the tube. These prior art structures are disclosed in the commonly assigned U.S. Pat. Nos. 4,480,210, 4,527,091 and 4,611,149. Devices incorporating the teachings of at least some of these patents are commercially available from the assignee of the present invention under the registered trademark KLYSTRODE.
The resonant coaxial cavity couples an input signal to an assembly including the cathode and grid. This resonant cavity has a length in the direction of the beam axis that is nominally either a half-wavelength at the frequency handled by the tube or a full wavelength at this frequency. In practice, it is most usually the latter.
The r.f. input signal to be amplified is transformer-coupled to the input resonant cavity which couples the field established in the cavity to the grid-cathode and grid-anode regions, in response to the input signal. In this document, the phrase "transformer coupled to the cavity" signifies that the r.f. power coming into or going out of a coaxial cable is coupled by r.f. magnetic fields to the cavity via loop coupling or by r.f. electric fields via probe coupling.